In computer systems, magnetic disk devices (such as an HDD and an FDD) and optical disk devices (such as an MO, CD, and DVD) are widely used to store data. However, those disk devices are large in volume and weight and consume large amounts of electric power. Furthermore, disk devices are precision machines that are very sensitive to mechanical vibrations or shocks. Therefore, disk devices are unsuitable for use as storage devices in information devices that are small in size, weight, and power consumption.
In recent years, to solve the above problems, semiconductor storage devices using a nonvolatile memory have been developed. The semiconductor storage device is a storage device using a flash EEPROM that is one type of non-volatile memory and that allows all data to be electrically erased, and the semiconductor storage device can be used in an interface similar to that used for conventional magnetic disk devices. Thus, the conventional magnetic disk device can be replaced by the semiconductor storage device.
Specific examples of semiconductor storage devices include an SD card, a COMPACT FLASH (registered trademark), a MEMORY STICK (registered trademark), SMART MEDIA (registered trademark), an xD-PICTURE CARD (registered trademark), and a USB memory used in a state in which it is inserted in a USB port of a host system.
Unlike magnetic disk devices or optical disk devices, semiconductor storage devices have no mechanical moving parts for performing a storing or reading operation. Thus, semiconductor storage devices are suitable for use in information devices having small size, weight, and power consumption. Because of the above-described advantages, the semiconductor storage devices can be loaded on information devices such as a digital camera, a digital video camera, or a portable telephone, to store image data, data received from a computer, or audio data. The data stored in the semiconductor storage device can be reproduced using proper devices.
Flash memories have a lifetime determined by the number of times rewriting/erasing is performed. In flash memories, writing of data is not allowed unless a memory area specified to be used to write data has already been erased. If rewriting/erasing is performed the maximum allowable number of times, which is in the range of 10,000 to 1000,000, it becomes impossible to continue using the flash memory. The tendency in recent years has been to reduce the number of times rewriting/erasing can be performed. To use the flash memory in a more useful manner, it is necessary to develop a way of reducing the number of times rewriting/erasing is performed or develop a new framework wherein flash memories are used as consumables.
In this regard, conventional semiconductor storage devices are used via an interface compatible with a conventional magnetic disk interface, and are dealt with by using a storage device that allows data to be randomly read and written in the same units as the magnetic disk sectors. Such an arrangement requires complicated processing associated with memory control. An effective method of reducing the number of times writing/erasing is performed is not known.
On the other hand, DVDs include various types of semiconductor memory devices such as a read-only type, a write-once type, and a rewritable type. In the write once type, data is written without rewriting physical blocks. If data is written until a memory space becomes full, it is not possible to either write additional data nor delete existing data to create a blank memory space to rewrite data (that is, write-once DVDs are consumables that are not rewritable). Low price DVDs can be used as archive memories for archiving data. Except for DVDs, semiconductor storage of various types, such a the read-only type, write-once type, and rewritable type, are not available at present.
In view of the above-described problems, it is a first object of the present invention to provide various types of semiconductor storage devices, such as the read-only type, the write-once type, and the rewritable type that can be used by a host system by using an existing command set or interface without needing any modification. A second object is to provide semiconductor storage devices of various types such as the read-only type, the write-once type, and the rewritable type using a semiconductor memory with the same structure. A third object is to provide a semiconductor storage device of the rewritable type in which intensive writing or erasing of data in one or more particular memory areas is prevented, thereby achieving a long lifetime. A fourth object is to provide a high-convenience semiconductor storage device of the write-once type that can be initialized when the memory capacity becomes full such that writing of data is again allowed until a blank memory area created by initialization becomes full. Although this semiconductor storage device is not of the write-once type in a strict sense, this semiconductor storage device is also referred to as a semiconductor storage device of the write-once type.